1. Field of the Invention
This invention relates to a method for developing an electrostatic image on an electrostatic image supporting member using a mono-component developer consisting of insulative magnetic toner.
2. Description of the Prior Art
In copiers which employ an electrophotographic process, a magnetic brush developing method using a two-component developer is commonly used as a method for developing an electrostatic image.
According to this method, while a sleeve having a magnet contained therein is being rotated, an electrostatic image supporting member is brushed with a brush-formed toner layer which is formed by attracting and holding magnetic developer on the sleeve to develop an electrostatic image. While this method provides a good developed image, it has drawbacks that fatigue of a carrier occurs and that the mixture ratio between the carrier and toner varies.
Meanwhile, in a method which employs a mono-component developer, the developer contains toner as a main component and contains no carrier particles. Therefore, such drawbacks as described above can be avoided. In order to attain such an advantage of the mono-component developer, several techniques have been already developed.
U.S. Pat. No. 4,121,931 discloses a mono-component developing method employing insulative magnetic toner. The insulative toner has a specific resistance of higher than 10.sup.12 .OMEGA.cm. An electric field which acts upon toner particles within a developing zone increases due to the presence of an electrical potential caused by a voltage source existing between a developer supporting member (hereinafter referred to as a sleeve) and an electrostatic image supporting member (hereinafter referred to as a drum) and other members and also to the presence of an electrical potential on the drum, so that, when the toner particles repetitively and turbulently contact a conductive portion of the developer supporting member, electric charges are injected onto the toner particles from the conductive face of the sleeve to charge the toner particles. In this instance, the amount of charges caused by friction is too little to contribute to the charging of the toner particles as a whole. According to this method, since the amount of charges caused by friction is very little, a strong electric field is required between the drum and the sleeve in order to provide toner particles with charges necessary for developing when a high density picture image is to be obtained. When the electric field is caused by a low electrostatic latent image potential, developing is difficult. Also, application of a high electric field will increase power consumption, and reduce the life of the drum.
In U.S. Pat. No. 4,259,427, magnetic toner has a specific resistance of 10.sup.14 .OMEGA.cm or more for insulation, and magnetic particles are arranged on a surface of each magnetic toner particle, so that the toner particles are charged by their mutual friction when they are transported. In this instance, the magnetite on a particle surface and the resin are mutually rubbed, so that the magnetite is charged negative while the resin is charged positive. As a result, each toner particle will have a number of positive and negative charges on a surface thereof. Some toner particles are charged positive while the other particles are charged negative. The positively charged particles and negatively charged particles present in a toner layer are cancelled by each other. When toner particles are charged by mutual friction thereamong, since there exist positively charged toner particles and negatively charged particles, toner particles will adhere to a countercharged portion of the drum of reverse polarity around an electrostatic image, causing useless toner consumption. Toner particles which adhere to the drum upon development may be used with an efficiency of 50 to 55 percent for a picture image on paper, and the remainder will remain unused and be scraped off the drum in a cleaning step. Further, since the magnetite protrudes from a surface of the resin in the toner, a photoconductor of Se or the like is readily damaged at a surface thereof, and hence such toner will naturally provide a limitation to a photoconductor to be used therewith. Further, if the picture image density is adjusted with a DC bias, toner will adhere to a non-image area. Accordingly, adjustment with a DC bias is not appropriate.
In German Patent Application Laid-Open Specification No. 2,361,418, magnetic toner has a specific resistance of 10.sup.10 to 10.sup.16 .OMEGA.cm, and toner particles are charged by injection of charges due to an electrical potential difference existing between a magnetic device on which toner particles are carried and a conductive donor surface which is opposed in a spaced relationship by a predetermined distance to the magnetic device. Then, the charged toner particles are partially transferred from the magnetic device to the conductive donor surface so as to allow the charged toner particles to contact a surface of an electrostatic image supporting member located in opposition to the conductive donor surface to effect developing.
An electrical potential difference applied between the magnetic device and a transfer zone of the conductive donor surface depends upon the specific resistance of toner and ranges from 3000 to 10000 V/cm. In other words, means for charging toner is injection of charges due to an electric field which is applied to the transfer zone between the magnetic device and the conductive donor surface. Similarly to the arrangement of U.S. Pat. No. 4,121,931, in order to charge toner particles, it is necessary to apply a high electric field to a transfer zone in which they exist, and hence, this arrangement presents similar drawbacks.
U.S. Pat. No. 3,909,258 discloses a mono-component developing method which employs conductive magnetic toner. In this method, in a process for developing an electrostatic image with toner, a conductive sleeve is electrically connected to a substrate of a drum on which an electrostatic image is carried so that particles of conductive and magnetic mono-component toner may be supported on a surface of the sleeve by a magnetic force. The drum is located adjacent and in opposition to the sleeve such that the electrostatic image on the drum is spaced by a predetermined fixed distance from the sleeve. An electric field intensity of the electrostatic image is selected such that an electrostatic force by charges of toner particles induced through an electric circuit by the electrostatic image is greater than the magnetic force, thereby enabling development. By the use of conductive toner, an electrostatic image can be easily developed irrespective of the polarity (positive or negative) thereof. However, since the toner is conductive, the sharpness of the image is readily deteriorated due to dispersion of toner particles when the image once developed is transferred, for example, to plain paper, and hence it is difficult to produce a good picture image. "Plain paper" here is defined as paper which is popularly used in present day copiers and which is not processed to have a resin coat for raising the electric resistance of the paper.